The present invention relates generally to efficient transfer of power from a piezo vibration harvester to a DC-DC converter, and more particularly to an improved DC-DC converter circuit for efficiently receiving a maximum amount of power from a piezo harvester.
Recently, various very low power integrated circuits that require extremely low amounts of operating current (often referred to as “nano-power” integrated circuits) have been developed which can be powered by very small amounts of power scavenged or harvested from ambient solar, vibrational, thermal, and/or biological energy sources by means of micro-energy harvesting devices. The harvested power then usually is stored in batteries or supercapacitors. (The term “nano-power” as used herein is intended to encompass circuits and/or circuit components which draw DC current of less than roughly 1 microampere.)
FIG. 1A shows an energy harvesting system 1-1 that includes a conventional piezo-electric harvester 2, an active rectifier circuit 3, and a DC-DC converter 4 for charging a battery or supercapacitor 6 and/or a load (not shown) which includes a switch control and PWM (pulse width modulation) circuit 9. Rectifier circuit 3 includes four switches S1-S4, two comparators A0 and A1, and two inverters 22 and 23. Active rectifier circuit 3 generates a harvested voltage Vhrv on conductor 18 which is applied to an input of switch control and PWM circuit 9 of DC-DC converter 4. DC-DC converter 4 generates an output voltage and output current which are supplied by conductor 5 to the battery 6. The (+) input of comparator A1 controls the control terminals of switches S1 and S2, and the (+) input of comparator A0 controls the control terminals of switches S3 and S4. As indicated in FIG. 1B, harvester 2 can be modeled as a parallel connection of a sinusoidal current source, an internal capacitance CPIEZO, and an internal resistance RPIEZO.
An optional filtering capacitor C0 may be connected between conductor 18 and ground. Piezo energy harvesters always have an output capacitance CPIEZO, which is not necessarily smaller than C0, depending on the brand or kind of harvester being used. Typically, DC-DC converter 4 in FIG. 1A is 80-90% efficient in transferring energy from conductor 18 to battery 6.
Referring to FIG. 2, the waveform represents the actual voltage VP(t) across piezo harvester 2. +Vhrv and −Vhrv are threshold voltages of the DC-DC converter, which may be determined by a maximum power point tracking (MPPT) circuit (not shown). For values of Vhrv less than +Vhrv, DC-DC converter 4 is in its “off” condition in which it does not convert Vhrv, and vibration energy is being wasted for recharging of the harvester output capacitance CPIEZO.
For values of the harvester output voltage VP(t) between +Vhrv and −Vhrv, energy generated by piezo harvester 2 is wasted by the charging and discharging of the capacitance CPIEZO. (Note that capacitor C0 is connected to the output of rectifier 3 and therefore is not charged and recharged by piezo harvester 2.) That energy is lost during the time interval between time t0 and t2 of transition B of the VP(t) waveform shown in FIG. 2. During the voltage levels +Vhrv and −Vhrv at the input of active rectifier 3, vibration energy recharges CPIEZO but that energy cannot be collected by DC-DC converter 4 and therefore is wasted. Piezo harvesters with the structure shown in FIG. 1A are able to actually collect less than ⅓ of the energy available from piezo harvester 2. See the article “A Comparison Between Several Vibration-Powered Piezo Electric Generators for Stand-Alone Systems” by E. Lefeuvere, A. Badel, C. Richard, L. Petit, and D. Guyomar, 2005, Science Direct, Sensors and Actuators A 126 (d006) 405-416, available online at www.sciencedirect.com; especially see FIGS. 6 and 7.
Before energy can enter harvester output capacitance CPIEZO and filter capacitance C0, the voltage VP(t) across piezo harvester 2 should reach the input threshold +Vhrv of DC-DC converter 4. At the end of the present vibration half-cycle and the beginning of the next one, the total harvester output capacitance, including CPIEZO, must be recharged to −Vhrv. This recharging energy (i.e., the subsequently mentioned CV2 energy) is supplied by the mechanical vibration source and the piezo harvester 2 receiving that vibration, but the recharging energy is wasted every vibration cycle.
Generally, in order to maximize power transfer from piezo harvester 2 into DC-DC converter 4, the equivalent output impedance of piezo harvester 2 should match the input impedance of DC-DC converter 2. The input impedance of DC-DC converter 4 is equal toZIN˜Vhrv/IL0(average),wherein IL0(average) is the average current through the inductor L0 of DC-DC converter 4. This means that the amplitude of Vhrv should be proportional to the vibration amplitude, and therefore it is not possible to minimize the amount of waste from the collected energy by choosing smaller Vhrv (because the amount of wasted energy CV2/2 is proportional to the square of the voltage across the capacitance).
To avoid having to waste the CV2 energy from piezo harvester 2 while switching its total output capacitance CPIEZO from +Vhrv to −Vhrv, a known technique can be used to increase the amount of energy collected from the piezo harvester. That technique is to connect a switch across piezo harvester 2 and briefly short-circuit it at time t0 in FIG. 2 until the voltage VP(t) goes through zero. This counterintuitive technique of dissipating collectible energy can improve the amount of charging of battery 6 because the amount of wasted power from piezo harvester 2 is reduced by a factor of 2. This avoids the need to waste the CV2 energy to recharge CPIEZO. Furthermore, use of a large inductor in series with the foregoing switch can further enhance the efficiency of power transfer from the piezo harvester to the battery.
Thus, there is an unmet need an improved circuit and method for extracting a maximum amount of power from a piezo energy harvester.
There is also an unmet need for an improved implementation of a piezo energy harvesting system that avoids the large amounts of power wasted in prior piezo energy harvesting systems.
There is also an unmet need for an improved implementation of a circuit and method for increasing the efficiency of a piezo energy harvesting system without use of additional switches and/or inductors.